1. Field of the Invention
Alcohols are conventionally produced by the direct vapor phase hydration of an olefin in the presence of a catalyst. The hydration reaction product contains the crude alcohol, unreacted olefin and by-product impurities. The reaction product is partially purified in a scrubbing zone wherein water is used to preferentially absorb the alcohol. The overhead from the scrubbing zone consists of unreacted olefin and most of the by-product impurities and is typically recycled to the reaction feed. The bottom stream from the scrubbing zone is the aqueous alcohol product which also contains a portion of the by-product impurities. This aqueous alcohol product is further processed in order to obtain a high purity alcohol product.
2. Description of the Prior Art
Alcohols are commercially synthesized by either the indirect hydration or direct hydration process. In a typical indirect process, the olefin is first absorbed in sulfuric acid; then, in a second step, water is added and an alcohol is formed. Direct hydration on the other hand is brought about by simultaneously contacting a solid or liquid catalyst with an olefin and water, thus producing alcohol in one step.
The principal use of the direct hydration method is the conversion of ethylene to ethanol. In one direct hydration process of the type described in U.S. Pat. Nos. 2,960,477 and Re 23,507, ethanol is produced at 540.degree. F and 1000 psia by passing a gaseous mixture of recycle ethylene, fresh ethylene, and water through a catalyst bed of celite impregnated with phosphoric acid. About 4 to 5% of the ethylene in the feed stream is converted to alcohol on each pass, thereby requiring a large ethylene recycle stream. The reactor effluent is partially condensed, and the resultant gas phase is scrubbed with water in a recycle gas scrubber at high pressures wherein the remaining alcohol vapor is adsorbed in the water while most of the gaseous unconverted olefin and the ether by-product that is formed in the reactor is recycled back to the reactor. The combined dilute ethanol solution from the liquid phase of the scrubbing step and the partial condensate stream are concentrated by stripping with steam and then the concentrated ethanol solution is hydrogenated at low pressure to convert the small amount of aldehyde impurities to alcohol. Small quantities of ether and light ends in the ethanol solution are then removed as distillates. The ethanol solution is further processed in a final distillation step where an ethanol-water azeotrope (95 vol % ethanol) is separated as a highly purified side stream. The azeotrope can be used directly, offered for sale or dehydrated to 100 vol % ethanol.
Along with the hydration reaction of ethylene with water to form ethyl alcohol, numerous side reactions take place in the reactor. Among the many by-products formed in the reaction are diethyl ether, acetaldehyde, crotonaldehyde, and butyl alcohol. However, the principal by-product is diethyl ether comprising from 1 to 10% of the ethanol produced. The diethyl ether produced in the reactor is more volatile than the ethanol and has little commercial value. Since the ethylene conversion to both ethanol and diethyl ether is equilibrium limited, it is possible to reduce the net diethyl ether yield to near zero by recovering the diethyl ether from the ethanol product stream in downstream facilities and recycling the diethyl ether back to the reactor. In the usual process sequence, the ethanol is recovered from the reactor product vapor, which is mainly unreacted ethylene, by cooling and then scrubbing the reactor product vapors with water. During the scrubbing step, a portion of the diethyl ether is also scrubbed out with the ethanol and passes with the aqueous ethanol product from the bottom portion of the recycle gas scrubber to a final purification step. However, most of the diethyl ether from the reactor product vapors remains with the unconverted ethylene and is recycled from the top of the recycle gas scrubber to the reactor where the diethyl ether suppresses additional diethyl ether due to the equilibrium reaction mechanism. Although usually less than five percent of the diethyl ether in the reactor product stream is scrubbed out with ethanol, this amount of diethyl ether nevertheless constitutes a significant yield loss if not recycled.
Various methods for the purification of ethanol are known in the art. Carrier, U.S. Pat. No. 2,648,711 is concerned with the recovery of alcohol free of ether from olefin hydration products by injecting steam in the recycle gas scrubber as a stripping agent and by operating the recycle gas scrubber under the same conditions of temperature and pressure as found in the reactor, thereby removing the ether overhead from the recycle gas scrubber together with all the unreacted hydrocarbons and recovering the alcohol as an aqueous solution bottoms product from the recycle gas scrubber. The feed stream to the recycle gas scrubber in Carrier enters near the middle of the column. The bottoms from the recycle gas scrubber are then stripped with steam in a separate final stripping column and the purified alcohol is recovered as a top product. One variation on the Carrier process is presented in DeJean et al, U.S. Pat. No. 3,265,594, wherein the inventor employs an oil sidedraw on the final stripping column to remove inpurities. Another variation on the Carrier process is taught by Ester, U.S. Pat. No. 3,156,629, wherein the crude alcohol product stream from the reactor and an alkaline-aqueous stream are both introduced near the top of the recycle gas scrubber such that the impurities and unconverted olefin are withdrawn overhead while the purified aqueous alcohol bottom product is further processed in a second distillation column where the final alcohol product is withdrawn as a sidestream.
In addition to the straight distillation techniques as shown above, hydrogenation and inorganic chemical processes are also used to purify the alcohols. Nommenson et al, U.S. Pat. No. 2,944,087, teaches a combination distillation-hydrogenation technique to purify alcohol. In the Nommenson patent, the initial recycle gas scrubber and final stripping column steps are similar to those taught in the Carrier patent. However, Nommenson teaches the additional step of catalytic hydrogenation of the alcohol product stream from the final stripping column thereby improving the odor and permanganate time of the purified alcohol product. Maycock et al, U.S. Pat. No. Re 23,507, is concerned with controlling the pH of the reactor, particularly to reduce the formation of higher unsaturated aldehydes such as crotonaldehyde and sorbaldehyde. The Maycock purification process still requires the subsequent use of distillation techniques as taught in the Carrier, DeJean or Ester patents.
In the aforementioned patents, at least some of the impurities in the alcohol stream are removed in various steps outside the reaction system--the reaction system being defined as that part of the direct hydration process comprising the reactor vessel and the initial recycle gas scrubber plus the associated heaters, exchangers, pumps, compressors, flash vessels, and lines. However, it is more attractive and economical to perform this removal within the reaction system, especially where the crude alcohol taken as a bottoms stream from the recycle gas scrubber is used directly as a chemical intermediate and the normal distillation to high purity is not essential.